1. Field of the Invention
The present invention relates to a surveying instrument having an optical distance meter.
2. Description of the Related Art
A conventional surveying instrument such as a total station has a function to measure the distance between two points and also horizontal and vertical angles. Such a conventional surveying instrument generally measures the distance between two points with a distance meter, usually an electronic distance meter (EDM) incorporated in or attached to the surveying instrument.
The electronic distance meter incorporates an optical distance meter which calculates the distance from the phase-difference or the time difference between a measuring light (externally-projecting light), which is projected toward a target, and an internal reference light. The optical distance meter includes a light-transmitting optical system for transmitting a measuring light to the target via the objective lens of a sighting telescope provided as a component of the electronic distance meter, and a light-receiving optical system for receiving light reflected by the target. The light-receiving optical system includes a wavelength selection filter for reflecting the measuring light which is reflected by the target to be passed through the objective lens of the sighting telescope, toward a light receiving element. The measuring light and the internal reference light are alternately incident on the light receiving element during operation of the optical distance meter.
To improve the precision of the optical distance meter by increasing the signal-to-noise ratio (S/N) of a signal of a received light, the light receiving element preferably receives only the measuring light and the internal reference light. To this end, much attempt has been made to narrow the range of wavelengths of the light which is allowed by the wavelength selection filter to reflect thereby. However, it is technically difficult to produce such a wavelength selection filter which reflects only the light with a specific wavelength corresponding to the wavelength of the measuring light, while a wide variation in the light wavelength range of such a wavelength selection filter is inevitable due to manufacturing error. Therefore, the cost of production increases if such a wavelength selection filter is mass-produced. Furthermore, there are manufacturing tolerances in the production of the laser diode (LD) which is used as a light source for emitting the measuring light, and the wavelength of the measuring light emitted by the laser diode varies due to a change in temperature. Therefore, if the range of wavelengths of the light which is allowed by the wavelength selection filter to pass therethrough is very narrow, the laser diode cannot cope with variation of temperature.
The present invention has been devised in view of the problems noted above, and accordingly, an object of the present invention is to provide a surveying instrument having an optical distance meter, wherein the range of wavelengths of the light which is incident on the light receiving element provided as an element of the optical distance meter can be narrowed.
Another object of the present invention is to provide a surveying instrument having an optical distance meter, wherein the laser diode which emits the measuring light is unaffected by variation of temperature.
To achieve the objects mentioned above, according to an aspect of the present invention, a surveying instrument is provided, including an optical distance meter which includes a light-transmitting optical system for transmitting a measuring light toward an object, and a light-receiving optical system for receiving light reflected by the object, the light-receiving optical system including a light-receiving element; a first wavelength selection filter and a second wavelength selection filter, both positioned in front of the light-receiving element, for allowing only light within a first wavelength range between a first wavelength and a second wavelength to pass therethrough, to be thereafter incident on the light-receiving element, wherein the first wavelength selection filter allows light with a wavelength equal to or greater than the first wavelength to pass therethrough, and a second wavelength selection filter allows light with a wavelength equal to or shorter than the second wavelength, which is longer than the first specific wavelength, to pass therethrough; and an angle adjusting device for adjusting an angle of inclination of each of the first wavelength selection filter and the second wavelength selection filter with respect to an optical path in which the first wavelength selection filter and the second wavelength selection filter are positioned.
In an embodiment, the surveying instrument further includes a sighting telescope, wherein the light-transmitting optical system transmits the measuring light via an objective lens of the sighting telescope, and the light-receiving optical system receives the light reflected by the object via the objective lens of the sighting telescope. The light-receiving optical system further includes a main filter which reflects only a portion of light, within a second wavelength range, which is reflected by the object and passed through the objective lens, wherein a remaining portion of the light reflected by the object and passed through the objective lens is passed through the main filter; and a reflecting element which reflects the portion of light reflected by the main filter to be eventually incident on the light-receiving element. The first wavelength range is narrower than the second wavelength range.
In an embodiment, the first wavelength selection filter and the second wavelength selection filter are respectively formed on a first filter plate and a second filter plate which are separate from each other, the angle adjusting device being provided for each of the first filter plate and the second filter plate.
In an embodiment, the first wavelength selection filter and the second wavelength selection filter are respectively formed on front and rear surfaces of a common filter plate, the angle adjusting device being provided for the common filter plate.
In an embodiment, the light-transmitting optical system includes a laser diode which emits the measuring light. The surveying instrument further includes a temperature sensor which senses a temperature of the laser diode, and a controller which controls the angle adjusting device to adjust the angle of inclination of at least one of the first wavelength selection filter and the second wavelength selection filter in accordance with the temperature sensed by the temperature sensor. According to this construction, the first and second wavelength selection filters can be automatically set to predetermined inclination angles in accordance with a variation in the wavelength of the measuring light emitted by the laser diode due to a change in temperature.
The measuring light can be light having a specific wavelength in the range of visible light or outside the range of visible light.
In an embodiment, the optical distance meter includes a collimating lens and a condenser lens, the first and second wavelength selection filters being positioned between the collimating lens and the condenser lens.
Preferably, the optical distance meter further includes an ND filter positioned in front of the collimating lens.
In an embodiment, the reflecting element is made of a parallel-plate mirror having front and rear surfaces parallel to each other, wherein the front surface faces the objective lens and is formed as a light transmitting mirror. The rear surface faces the main filter and is formed as a light receiving mirror.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-132204 (filed on May 1, 2000) which is expressly incorporated herein by reference in its entirety.